the term baker’s dozen has a pretty ancient origin? What we mean with it is any group of 13 (of baked products), i.e. a dozen plus one. But how did it come to be?

It is thought to come from the former practice among bakers and other tradespeople in medieval England of giving 13 items to the dozen as a safeguard against penalties for short weights and measures. Thing is, you might even get a beating if your pieces of bread were too small. That is, weighted too little, for as a smart baker you would want to use less flour.

So, to compensate for that, the baker would put in an extra piece just to be on the safe side. Or sometimes even two (extra pieces or loafes) just to be extra sure.